The invention relates to an arrangement for reducing the effect of radio interference upon a receiving antenna provided upon the windshield of an automotive vehicle when the windshield is also provided with heating conductors arranged in the vincinity of the antenna.
It is known that such heating conductors produce high-frequency interference fields in their general vicinity, due to the fact that they conduct not only heating current but also interference currents. These interference currents are generated, for example, as a result of the operation of the ignition system of the engine of the vehicle, and are transmitted via the current supply lines of the vehicle's electrical system into the heating conductors on the windshield or rear window.
This problem will be explained with reference to the schematic depiction in FIG. 1. FIG. 1 depicts a glass pane, such as a windshield or rear window of a vehicle, in a window frame 2. The heating conductors form upon the window 1 a layer 3 made up of a single continuous and substantially transparent conductor, or made up of a plurality of discrete conductors. The conductor 3 carries current which results in the establishment of a magnetic field having field lines H. At a certain distance away from the conductive layer 3 the shape of the field is substantially as indicated in FIG. 1, and this field includes as constituent components the radio interference fields referred to above. If for example the receiving antenna is constituted by a conductor 4, the magnetic interference fields surrounding the conductor 4 induce interference voltages in the antenna conductor. Also emanating from the heating conductor layer 3 are electrical field lines E which have approximately the shape shown in FIG. 1 and likewise include interference fields as constituent components. Where the field lines of these interfering electrical fields intersect the antenna conductor 4, the displacement currents associated with these electrical fields will induce interference currents in the receiving antenna.
Accordingly, the interference fields from the heating conductors will interfere with the reception of high-frequency signals by the vehicle antenna, if the antenna is located in the vicinity of the heating conductors. This is the case to a particularly significant extent, for example, when the rear window of the vehicle is provided with both a large heating conductor system and also with receiving antenna conductors.
It is known to counteract this difficulty by providing filters in the supply lines for the heating conductor, for example choke coils for reducing interference currents in the heating conductors. However, this known expedient is effective only at relatively high frequencies and only over very limited bandwidths.